The basic principle of an optical gaging instrument is to provide a method to inspect a desired feature with versatile way of non-contact gaging. This enables manufacturers the ability to inspect a wide variety of parts without the need for complex fixtures. This non-contact method of inspection allows an inspector to view a desired feature using the magnification ability of optics.Optical inspection instruments allow to determine characteristics such as  size, shape, location or surface finish.

The first type of optical inspection instruments were developed to enhance the ability of the human eye to see certain aspects of an object. These simple lenses magnified an image that allowed the user to discern features unable to be seen by the naked eye. These basic magnifiers allowed an inspector to view and compare an object but did not allow the object or feature to be accurately measured.

Accurate measurement requires that the object or feature being observed be compared to a standard of known size. The first application of this principle was called a shadow graph. This instrument used a lamp to project a two dimensional image of the object being observed on a flat surface. The shadow could then be measured with a known standard, for example - a ruler to determine its size. By placing a magnification lens in the path of the image being projected the inspector was able to enhance or magnify the image that enabled greater clarity of certain features.

These simple shadow graphs often projected the image onto a grid of known size where the operator could determine the size of an object by simply counting the number of squares the shadow of the object occupied and then dividing that by the magnification of the objective lens. Overlay charts were also developed which allowed the operator to compare the shadow projected to a picture drawn of the feature being inspected. Often this overlay was two images drawn to allow rapid determination of a feature size or shape. One image drawn to the minimum allowable size of the part's tolerance while the other was drawn to the features maximum allowable size. The operator could quickly determine whether the part is good or bad by simply inspecting the screen to insure that the shadow is between these two lines. This type of comparison allowed an inspector to determine if a part is in or out of tolerance but was not readily adaptable for inspecting a wide variety of parts and shapes.

Manual micrometer heads were then fitted to stages or tables that slid along the horizontal and vertical axes of the machine. This allowed the versatility to directly inspect feature size or location without the use of a special overlay chart or grid. The inspector would line up one edge of the shadow being projected on a screen cross line and then move the stage micrometer to the other end of the shadow. The distance that the stage traveled is the dimension of the part. With this principle applied to both X and Y axes optical comparators were able to measure any two dimensional feature that is projected on the screen.

The major turning point in optical gaging came about when instrument manufacturers replaced manual micrometer heads with electronic encoders and digital readouts. Electronic encoders mounted to the stage sent a signal to a digital readout that decodes the signal and displays the distance and direction that the stage traveled. The first digital readouts simply displayed the X and Y location of the stage.

With the application of microprocessor technology to the digital readout, geometric principles are applied to calculate features such as angles, radii and relative position to further enhance the capabilities of the optical instrument. At about this same time video technology was developed which allowed the replacement of the standard projection system with a lens and camera system. The part being inspected is digitized, manipulated and displayed on a TV style monitor. Recent developments and improvements in this technology have seen the ability to measure in all three dimensions as the “Z” or focus axis can now be automatically controlled.